An earth sensor is a critical component of a communication or remote sensing satellite's attitude control subsystem. It is generally desirable to maintain a communications or remote sensing satellite in a geosynchronous orbit about the earth so as to enable a communication beam or sensor field of view from the satellite to accurately cover a desirable area, such as a particular country, on the surface of the earth. Any deviations from this orbit will alter the coverage of the beam or view. Accordingly, satellites are provided with sensors for sensing changes in orientation of the satellite relative to the earth.
One prior art earth sensor relies on infrared radiation focused on thermally sensitive detectors. Another relies on the intensity of visible and ultraviolet radiation from the earth disk. The earth as seen from space does not always have the same shape in the visible light spectrum. It is therefore difficult to design a video earth sensor which accommodates this characteristic without providing prohibitively large and complex data processing capabilities.